Cornell University1 Cornell Tech2 Abstract How can we tell whether an image has been mirrored? While we understand the geometry of mirror reflections very well, less has been said about how it affects distributions of imagery at scale, despite widespread use for data augmenta- tion in computer vision. In this paper, we investigate how the statistics of visual data are changed by reflection. We refer to these changes as “visual chirality,” after the concept of geo- metric chirality—the notion of objects that are distinct from their mirror image. Our analysis of visual chirality reveals surprising results, including low-level chiral signals pervad- ing imagery stemming from image processing in cameras, to the ability to discover visual chirality in images of people and faces. Our work has implications for data augmentation, self-supervised learning, and image forensics. 1. Introduction “...there’s a room you can see through the glass—that’s just the same as our drawing room, only the things go the other way.” — Lewis Carroll, “Alice’s Adventures in Wonderland & Through the Looking-Glass” There is a rich history of lore involving reflections. From the stories of Perseus and Narcissus in ancient Greek mythol- ogy to the adventures of Lewis Carroll’s Alice and J.K. Rowl- (a) (b) (c) Figure 1. Which images have been mirrored? Our goal is to understand how distributions of natural images differ from their reflections. Each of the images here appears plausible, but some subset have actually been flipped horizontally. Figuring out which can be a challenging task even for humans. Can you tell which are flipped? Answers are in Figure 2. less about how it changes what we learn from that data—this, despite widespread use of image reflection (e.g., mirror-flips) for data augmentation in computer vision. This paper is guided by a very simple question: How do the visual statistics of our world change when it is reflected? One can understand some aspects of this question by con- sidering the images in Figure 1. For individual objects, this question is closely related to the concept of chirality [12]. An object is said to be chiral if it cannot be rotated and translated into alignment with its own reflection, and achiral otherwise.1 Put differently, we can think of chiral objects as being fundamentally changed by reflection—these are :2006.09512v1 [cs.CV] 16 Jun 2020
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